Exercise can be defined as any activity which requires a significant amount of exertion, either physical or mental. Mental exercise affects the brain, and can change the mental capabilities of the exerciser if done over a significant period of time. Physical exercise affects the physical body through movement of the body. There are two primary groups of physical exercise which can be performed by a person; aerobic, and anaerobic. Aerobic exercise focuses on improving the cardiovascular system and increasing endurance, such as swimming, running, cycling, and rowing. Anaerobic exercise focuses on strengthening the muscles which can increase muscle mass and thereby increase the amount of force the muscles are capable of applying to an object. Some good examples of anaerobic exercises include lifting weights, pushups, pull-ups, crunches, and reverse crunches.